Ethylene (ethene) plants that crack liquid feeds produce cracked gases, pyrolysis gas oil and heavy pyrolysis fuel oil at high temperatures. This mixture passes through an oil quench tower (also known as primary fractionator or gasoline fractionator) where gases (C9 and lighter) are cooled and separated from the heavy oils. The lighter separated material, rich in unsaturated hydrocarbons, is known as raw gasoline or py-gas oil. Py-gas oil is refluxed in the upper section of the oil quench tower and its counter current flow cools cracked gases.
As the py-gas oil is subjected to heat, it increases in viscosity and the heavier components drop to the bottom section of the oil quench tower, leading to an increase in the viscosity of the hydrocarbon present in the bottom section of the tower and fouling. This is possibly as a result of polymerization of the unsaturated hydrocarbon components. Viscosity increase and fouling is problematic in that it can adversely affect the quality of the final product.
In an attempt to reduce viscosity in the bottom section of the tower, light cycle oil (LCO), and/or py-gas oil may be added to the tower, thereby reducing the viscosity by dilution. However, this procedure results in considerable expense for the plant operators. Therefore, other methods of preventing a viscosity increase have been proposed.
Various methods of chemical treatment have been proposed to prevent viscosity increase during ethylene production. These include the use of sulfonic acids or salts as proposed in U.S. Pat. No. 5,824,829 to Maeda et al. (“Maeda”) and the use of phenylenediamines. It has been proposed to add these compositions to a hydrocarbon stream in order to prevent an increase in viscosity. However, while these compositions have been suggested to be inhibitors of polymerization, they generally are used in combination with other chemical treatments or in combination with the addition of py-gas oil or LCO to adequately prevent the increase of viscosity of the hydrocarbon mixtures.
Another method of mitigating fouling and reducing viscosity is proposed in U.S. Pat. No. 5,985,940 to Manek et al. (“Manek”). Manek proposes the use of mono- and/or polyalkyl-substituted phenol-formaldehyde resins.
Although polymerization of the components in the oil quench tower contributes to the increase of viscosity in the bottom section, compositions that inhibit the polymerization of a particular monomer do not necessarily prevent a viscosity increase in an oil quench tower or during ethylene production. This is demonstrated by examples of known vinyl monomer polymerization inhibitors that are ineffective in quench oil applications. One reason for this observation is that the hydrocarbons present in the bottom of the oil quench tower are a mixture of a variety of different monomers and other components. For example, these include a variety of compounds including a variety of unsaturated compounds, such as unsaturated aromatics, including, without limitation, styrene, methyl styrene, divinylbenzene, and indene.
Therefore, there is a need for other methods of inhibiting fouling and/or viscosity increase that provides an adequate results. Desirably, the method may be used during the operation of an ethylene plant and will provide a more cost-effective manner of preventing viscosity increase and fouling.